Re: [PATCH v6 2/2] Add user-mode only option to unprivileged_userfaultfd sysctl knob

From: Lokesh Gidra <lokeshgidra@google.com>
To: Kees Cook <keescook@chromium.org>,
	Jonathan Corbet <corbet@lwn.net>, Peter Xu <peterx@redhat.com>,
	Andrea Arcangeli <aarcange@redhat.com>,
	Sebastian Andrzej Siewior <bigeasy@linutronix.de>,
	Andrew Morton <akpm@linux-foundation.org>
Cc: Alexander Viro <viro@zeniv.linux.org.uk>,
	Stephen Smalley <stephen.smalley.work@gmail.com>,
	Eric Biggers <ebiggers@kernel.org>,
	Daniel Colascione <dancol@dancol.org>,
	"Joel Fernandes (Google)" <joel@joelfernandes.org>,
	Linux FS Devel <linux-fsdevel@vger.kernel.org>,
	linux-kernel <linux-kernel@vger.kernel.org>,
	linux-doc@vger.kernel.org, Kalesh Singh <kaleshsingh@google.com>,
	Calin Juravle <calin@google.com>,
	Suren Baghdasaryan <surenb@google.com>,
	Jeffrey Vander Stoep <jeffv@google.com>,
	"Cc: Android Kernel" <kernel-team@android.com>,
	Mike Rapoport <rppt@linux.vnet.ibm.com>, Shaohua Li <shli@fb.com>,
	Jerome Glisse <jglisse@redhat.com>,
	Mauro Carvalho Chehab <mchehab+huawei@kernel.org>,
	Johannes Weiner <hannes@cmpxchg.org>,
	Mel Gorman <mgorman@techsingularity.net>,
	Nitin Gupta <nigupta@nvidia.com>,
	Vlastimil Babka <vbabka@suse.cz>,
	Iurii Zaikin <yzaikin@google.com>,
	Luis Chamberlain <mcgrof@kernel.org>,
	"open list:MEMORY MANAGEMENT" <linux-mm@kvack.org>
Subject: Re: [PATCH v6 2/2] Add user-mode only option to unprivileged_userfaultfd sysctl knob
Date: Thu, 19 Nov 2020 19:10:20 -0800	[thread overview]
Message-ID: <CA+EESO74RWq+FxV5H8BS6vb0E+icbzU6fYMEAbiFBB79FHiHVg@mail.gmail.com> (raw)
In-Reply-To: <20201120030411.2690816-3-lokeshgidra@google.com>

On Thu, Nov 19, 2020 at 7:04 PM Lokesh Gidra <lokeshgidra@google.com> wrote:
>
> With this change, when the knob is set to 0, it allows unprivileged
> users to call userfaultfd, like when it is set to 1, but with the
> restriction that page faults from only user-mode can be handled.
> In this mode, an unprivileged user (without SYS_CAP_PTRACE capability)
> must pass UFFD_USER_MODE_ONLY to userfaultd or the API will fail with
> EPERM.
>
> This enables administrators to reduce the likelihood that an attacker
> with access to userfaultfd can delay faulting kernel code to widen
> timing windows for other exploits.
>
> The default value of this knob is changed to 0. This is required for
> correct functioning of pipe mutex. However, this will fail postcopy
> live migration, which will be unnoticeable to the VM guests. To avoid
> this, set 'vm.userfault = 1' in /sys/sysctl.conf.
>
> The main reason this change is desirable as in the short term is that
> the Android userland will behave as with the sysctl set to zero. So
> without this commit, any Linux binary using userfaultfd to manage its
> memory would behave differently if run within the Android userland.
> For more details, refer to Andrea's reply [1].
>
> [1] https://lore.kernel.org/lkml/20200904033438.GI9411@redhat.com/
>
> Signed-off-by: Lokesh Gidra <lokeshgidra@google.com>
> Reviewed-by: Andrea Arcangeli <aarcange@redhat.com>
> ---
>  Documentation/admin-guide/sysctl/vm.rst | 15 ++++++++++-----
>  fs/userfaultfd.c                        | 10 ++++++++--
>  2 files changed, 18 insertions(+), 7 deletions(-)
>
> diff --git a/Documentation/admin-guide/sysctl/vm.rst b/Documentation/admin-guide/sysctl/vm.rst
> index f455fa00c00f..d06a98b2a4e7 100644
> --- a/Documentation/admin-guide/sysctl/vm.rst
> +++ b/Documentation/admin-guide/sysctl/vm.rst
> @@ -873,12 +873,17 @@ file-backed pages is less than the high watermark in a zone.
>  unprivileged_userfaultfd
>  ========================
>
> -This flag controls whether unprivileged users can use the userfaultfd
> -system calls.  Set this to 1 to allow unprivileged users to use the
> -userfaultfd system calls, or set this to 0 to restrict userfaultfd to only
> -privileged users (with SYS_CAP_PTRACE capability).
> +This flag controls the mode in which unprivileged users can use the
> +userfaultfd system calls. Set this to 0 to restrict unprivileged users
> +to handle page faults in user mode only. In this case, users without
> +SYS_CAP_PTRACE must pass UFFD_USER_MODE_ONLY in order for userfaultfd to
> +succeed. Prohibiting use of userfaultfd for handling faults from kernel
> +mode may make certain vulnerabilities more difficult to exploit.
>
> -The default value is 1.
> +Set this to 1 to allow unprivileged users to use the userfaultfd system
> +calls without any restrictions.
> +
> +The default value is 0.
>
>
>  user_reserve_kbytes
> diff --git a/fs/userfaultfd.c b/fs/userfaultfd.c
> index 605599fde015..894cc28142e7 100644
> --- a/fs/userfaultfd.c
> +++ b/fs/userfaultfd.c
> @@ -28,7 +28,7 @@
>  #include <linux/security.h>
>  #include <linux/hugetlb.h>
>
> -int sysctl_unprivileged_userfaultfd __read_mostly = 1;
> +int sysctl_unprivileged_userfaultfd __read_mostly;
>
>  static struct kmem_cache *userfaultfd_ctx_cachep __read_mostly;
>
> @@ -1966,8 +1966,14 @@ SYSCALL_DEFINE1(userfaultfd, int, flags)
>         struct userfaultfd_ctx *ctx;
>         int fd;
>
> -       if (!sysctl_unprivileged_userfaultfd && !capable(CAP_SYS_PTRACE))
> +       if (!sysctl_unprivileged_userfaultfd &&
> +           (flags & UFFD_USER_MODE_ONLY) == 0 &&
> +           !capable(CAP_SYS_PTRACE)) {
> +               printk_once(KERN_WARNING "uffd: Set unprivileged_userfaultfd "
> +                       "sysctl knob to 1 if kernel faults must be handled "
> +                       "without obtaining CAP_SYS_PTRACE capability\n");
>                 return -EPERM;
> +       }
>
>         BUG_ON(!current->mm);
>
> --
> 2.29.0.rc1.297.gfa9743e501-goog
>
Adding linux-mm@kvack.org list